New Flash library

I wrote a new library, Flash, which abstracts away most of the complexity of PROGMEM programming. It provides String, Array, and Table types that make ROM-based data collections as easy to use as “normal” types. Each overrides the C++ [] operator, so to get at individual elements, one simply uses familiar array syntax. For example, if you have 1000 floating point temperatures in a flash-based “temperature_table” array, you might write code like this: